Java Doc for InstructionStack.java in  » Database-DBMS » db4o-6.4 » EDU » purdue » cs » bloat » inline » Java Source Code / Java DocumentationJava Source Code and Java Documentation

This class is an InstructionVisitor that updates the instruction stack representation appropriately. When there is a merge in control flow, two InstructionStacks are merged using the merge method.

This class is also used to determine whether the object at a given stack depth is "preexistent". An object preexists if we can guarantee that it was created outside of the method in which it is used. While it is possible to determine which fields are preexistent (see "Inlining of Virtual Methods" by Detlefs and Ageson in ECOOP99), we only keep track of local variables that preexist.

We determine which local variables preexist as follows. Initially, only the local variables for method parameters preexist. When a store is encoutered, we determine if the set of instructions on top of the stack consist of loads preexistent variables. If so, then the variable being stored into is preexistent. However, objects that are the result of an allocation (constructor call) in the method are preexist. Thus, we maintain the preexist information as a set. If the set is null, then the object does not preexist. If the set is empty, then it preexists and came from at least one argument. If the set is non-empty, then it contains the type(s) of the constructor(s) from which it originated. Pretty neat, huh?